1. Field of the Invention
This invention relates in general to methods and apparatus for measuring the path length in electrical communication transmission systems.
2. Description of the Prior Art
German Offenlegungsschrift No. 2,613,054 which has been filed in the United States Patent Office on Feb. 22, 1977, Ser. No. 770,266 discloses a circuit arrangement for the correction of the damping curve of a communication transmission path in which a test signal generator produces a test signal which consecutively different individual frequencies. A detector is provided at the test signal receiver and the detector detects the arrival of one of the individual frequencies used for measurement and conveys this information to a counter. This counter counts by one step further each time that an individual frequency arrives and actuates the corresponding memory devices which change the frequency of a superposition oscillator for each of the individual frequencies such that the converted intermediate frequencies have the same value. A very narrow band intermediate frequency filter can thus be employed at the receiver. The respective frequency values of each memory device is simultaneously determined. The amplitude value measured at one individual frequency together with the frequency value is available for evaluation.
It must be possible to adjust the transmission characteristics measurement methods operating with a number of individual frequencies in the total transmission range or in a partial range thereof in order to determine the respective conditions of the individual transmission paths for each frequency. As the communication transmission may not normally be interfered with, it is difficult to make operational measurements at the individual frequencies relative to their number and their distribution on the transmission frequency band and this must be adjusted to the respective operating conditions of the band such as channel width, pilot channels, control channels and so forth. Thus, in a specific transmission system, one can only measure with 10 different individual frequencies, however, in another different transmission system, the operation may be accomplished with 25 individual frequencies. If one utilizes counter circuits in the known manner at the receiver difficulties can arise in specific operating conditions in that the counter circuit may exhibit an incorrect counter value due to the failure of an individual frequency. For this instant on, all measuring values are wrongly assigned to the subsequent frequencies because the counter does not register the failure of the one individual frequency and from then on indicates a value which is too low for the counter value.